1. Field of the Invention
The present invention relates to a lens, and more particularly to a zoom lens.
2. Description of Related Art
To attain the advantages of wide angle and small size, a zoom lens is normally designed to have two lens groups in the conventional technique. A first lens group, which is closer to an object side, has a negative refractive power and a second lens group has a positive refractive power. In other words, the first lens group is used for diverging light beams and the second lens group is used for converging light beams.
However, an f-number of the conventional wide-angle zoom lens comprising two lens groups is greater than 3.5. Hence, it is difficult to reach a large relative aperture. In addition, eliminating distortion is difficult while this type of zoom lens is in a wide-end disposition. The conventional technique usually relies on two methods for minimizing the distortion. One of the methods includes distancing the positive lens in the first lens group or the negative lens in the second lens group away from the aperture stop. However, this method not only increases the dimension of the first lens group or the second lens group, but overall length of the zoom lens also increases as well so that it is harder to form a compact structure. The other method is to increase the refractive power of the positive lens in the first lens group or the negative lens in the second lens group. If the refractive power of the positive lens in the first lens group is increased, the refractive power of the other negative lenses in the first lens group must be increased. Hence, high-level distortion aberration and coma aberration is produced leading to a deterioration of overall image quality. A similar condition occurs when the refractive power of the negative lens in the second lens group is increased.